Natural gas is a major and attractive energy source in the United States and other countries which is distributed on a wide scale by means of pipelines to the ultimate users. The composition of natural gas at the wellhead varies widely from field to field and many components may need to be removed by processing before delivery to the pipeline, as noted in Encyclopedia of Chemical Technology, Kirk-Othmer, Third Edition, Vol. 1, pp. 630-652, John Wiley & Sons, New York, 1978. Condensate must be removed from wet gas, both to eliminate the possibility of condensation in the pipeline itself and to separate these components as another marketable product. Water vapor must also be removed to preclude the formation of gas hydrate and hydrogen sulfide is removed because of its poisonous and corrosive characteristics. Other components such as carbon dioxide and helium may also require removal if they are present.
A number of different processing techniques have been developed for the removal of specific contaminants in the gas and depending upon the composition of the wellhead gas, one or more of the specific processing techniques may be employed at any given gas production site. Water, for example, is generally removed either by absorption in a hydroscopic liquid or adsorption on an activated solid dessicant. Acid gases such as carbon dioxide, hydrogen sulfide and other sulfur compounds may be removed by processes such as the Sulfinol and Selexol processes or by alkali scrubbing, normally using an organic scrubbing agent such as monoethanolamine, diethanolamine or triethanolamine. Natural gas liquids may be recovered by oil absorption or, more recently, by the cryogenic turboexpander process.
Regardless of the specific processing sequence used to condition the gas for pipelining, it is normally passed through a number of items of process equipement prior to removal of the water which generally is present in all wellhead gas. Conventionally, the gas will pass through filters to remove entrained solid contaminants as well as compressors and other items of process equipment employed in the purification train. One problem which is encountered with these items of equipment which precede the water removal is that mineral species and other contaminants from the producing formations may be dissolved in or entrained with the water and may contaminate the equipment with which it comes into contact. Among the contaminants of this kind are dissolved salts including, especially, mineral species of low solubility which may deposit as scale on the filters and other surface equipment. Among the minerals of this kind which cause a particular problem are the sulfates of the alkaline earth metal, including, in particular, calcium, strontium and barium.
As these mineral species precipitate on the surfaces of the water-carrying or water-containing systems, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
The scale deposits formed from the sulfates of the alkaline earth metals including barium and strontium present a unique and particularly intractable problem. Under most conditions, these sulfates are considerably less soluble in all solvents than any of the other commonly encountered scale-form ng compounds, as shown by the comparative solubilities given in Table 1 below.
TABLE 1 ______________________________________ Comparative Solubilities, 25.degree. C. in Water. Scale Solubility, mg./l. ______________________________________ Gypsum 2080.0 Strontium sulfate 140.0 Calcium Carbonate 14.0 Barium sulfate 2.3 ______________________________________
Barium sulfate scale is extremely difficult to remove chemically, especially within reasonably short periods of time: the solvents which have been found to work generally take a long time to reach an equilibrium concentration of dissolved barium sulfate, which itself is usually of a relatively low order. Consequently, barium sulfate must be removed mechanically or the equipment, e.g. pipes, etc., containing the deposit must be discarded.
Another problem associated with the formation of barium and strontium sulfate scales is that radium, another member of the alkaline earth group of metals, tends to be deposited at the same time in the form of an insoluble sulfate so that the equipment becomes radioactive, and may eventually become unusable for safety reasons alone. The principal isotopes of radium which have been identified in these scale deposits are radium-226 and radium-228, with half-lives of 1600 years and about 5.8 years, respectively, so that there is a considerable potential hazard if they are allowed to remain on the equipment. Radioisotopes of metals such as thallium and thorium e.g. thallium-208, thorium-232 and other radioactive isotopes--frequently formed by the decay of uranium--may also be deposited with the radium with the same result. This problem is encountered with gas processing equipment where the filters and dehydration media, including screens, sorbent beds and media e.g. silica, charcoal or alumina may become quite radioactive with extended use. Since these radioactive elements are embedded in the scale which itself can be removed only with difficulty, the decontamination of the equipment and material is a substantial problem.